This project investigates the neural mechanisms that subserve the monkey's ability to detect innocuous cutaneous stimuli (air puffs) and noxious stimuli. The magnitude of sensations produced by small increases in air puff stimuli was studied with use of a reaction time paradigm. The monkey initiated a trial by pressing an illuminated button. Subsequently air puff stimuli (AP1) of identical intensity were delivered to the face at the rate of one per second. After a variable time period between 4 and 10 seconds, an air puff of higher intensity (AP2) was presented. The subject was required to release the button as soon as the larger air puff stimuli was detected. Discrimination speed was defined as the reciprocal of the time interval between the onset of the larger air puff stimulus and the release of the button. The discharge of medullary dorsal horn neurons was recorded while the monkey performed the air puff psychophysical task. A subpopulation of wide-dynamic-range (WDR) neurons were found that encode the intensity of innocuous air puff stimuli with an increase in peak discharge frequency. In these neurons, the peak discharge frequency for AP2 is strongly correlated with the animals detection speed. An additional subpopulation of low threshold mechanoreceptive (LTM) neurons was found that encoded the intensity of air puff stimuli and their discharge was highly correlated with the animals detection speed. There was no significant difference between the encoding properties of WDR and LTM neurons. We therefore conclude that subpopulations of both WDR and LTM neurons in the trigeminal nucleus caudalis of the primate can account for the monkey's ability to discriminate innocuous air puff stimulation. In a second study, we examined the changes in receptive field properties of nociceptive neurons in the primary somatosensory cortex (SI) after an intradermal injection of capsaicin. In humans, an intradermal injection of capsaicin produces burning pain, cutaneous hyperalgesia and allodynia. Neurons that responded maximally to a pinch stimulus received an intradermal injection of capsaicin. After the injection of capsaicin, the discharge rate of all neurons increased dramatically and remained elevated for an average of 75 seconds. The injection also produced an expansion in receptive field size in 22 of 30 neurons. The expansion (10-1500%) was present within 5 min. and persisted for at least 25 min. The responses to a series of graded mechanical stimuli also increased after capsaicin injection with the largest relative increase to the pressure stimulus. These results indicate that nociceptive neurons in primate SI cortex respond to intradermal injection of capsaicin and that tonic activity in nociceptive pathways can alter receptive field size and responses to mechanical stimulation. We conclude that many of the receptive field changes seen in nociceptive SI neurons can account for the burning pain, cutaneous hyperalgesia and allodynia in humans after injection of capsaicin.